Signal acquisition devices or test and measurement devices such as digital storage oscilloscopes (DSOs) and the like receive one or more signals under test (SUT) via one or more respective input channels. It is important that the input channels be electrically isolated from each other and from the chassis ground of the test and measuring device. Such isolation is necessary for signals having a frequency from DC to the bandwidth of the measuring device. Such isolation allows multiple signals under test to be provided to, and analyzed by, the same measuring device without affecting the systems/signals under test.
One isolation method known to those skilled in the art for use in wide bandwidth systems is the so-called “two path scheme” in which an input signal is broken up into two signals; namely, a low frequency input signal and a high frequency input signal. Optocouplers are used for low frequency signals (e.g., signals under 1 MHz) and wide band transformers are used for the high frequency signals. Unfortunately, optocouplers are difficult and expensive to linearize, and wide band linear transformers are expensive. Additionally, it is difficult to insure that the resulting isolated low end high frequency signal components of the signals under test are sufficiently well matched such that they may be recombined to produce a “flat” signal suitable for further processing by the test and measurement device.